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Diatomaceous Soil Deposits: Geological Features, Geotechnical Properties and Associated Engineering Problems

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Elaine C. G. Barreto and Tácio M.P. de Campos

Submitted: 16 May 2026 Reviewed: 08 June 2026 Published: 25 August 2026

DOI: 10.5772/intechopen.1016638

Sediment Transport and Depositional Processes IntechOpen
Sediment Transport and Depositional Processes Edited by Gemma Aiello Ambrosino

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Sediment Transport and Depositional Processes [Working Title]

Ph.D. Gemma Aiello Ambrosino

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Abstract

Diatoms are unicellular algae of the class Bacillariophyceae that grow in sunlit, saline, brackish, or freshwater environments and consume dissolved silica to build their skeletons, forming frustules that make them microfossils. In recent years, the sedimentary deposits containing diatom microfossils have been increasingly explored in publications across the engineering geology and geotechnical engineering fields, especially regarding experimental laboratory research, drawing attention to this type of geological material, its physical characteristics, and the effects on the hydro-mechanical properties of these soils and their probable impacts in the field. In situ, it is generally found in locations associated with imminent geological risks, such as tectonically active zones, excessive settlements, or slopes with the potential for abrupt and large-scale ruptures and landslides. Research involving field tests or instrumentation reports in structures founded on deposits is less frequent. In this context, this comprehensive literature review chapter examines the main aspects of diatomaceous soil deposits, including their geological origins and worldwide occurrence, formation processes, and microstructural characteristics. Additionally, morphological characteristics and geotechnical properties of the diatom microfossil particles, and basic considerations regarding their use as input for product manufacturing are discussed. Furthermore, geotechnical properties of these soils and some examples of associated geotechnical problems and geological accidents, such as landslides and foundation problems in construction projects on them, are mentioned. In fact, diatomaceous soil deposits are intriguing and pose challenges for proper geotechnical engineering understanding, being potentially classified as a difficult or problematic soil type, requiring further basic research and a case-by-case approach in practice.

Keywords

  • diatoms
  • diatomaceous soils
  • microstructure
  • geotechnical properties
  • engineering behaviour

1. Introduction

This comprehensive literature review chapter on diatomaceous soil deposits examines basic considerations when comparing them with the challenges of researching them, using them as input for product manufacturing, or constructing a structure on this geological material.

With diverse applications, the geological material composed of diatom microfossils has been used as a filter and abrasive agent in the manufacture of paints and beer, and as a porous medium for environmental decontamination.

Sedimentary deposits containing diatom microfossils have been increasingly explored in publications across geology and engineering, especially in experimental laboratory research, drawing attention to this type of geological material, its physical characteristics, and the effects on the physical and mechanical properties of these soils and their probable impacts in the field. In situ, it is generally found in locations associated with imminent geological risks, such as tectonically active zones, excessive settlements, or slopes with the potential for abrupt and large-scale ruptures and landslides. Research involving field tests or instrumentation reports in structures founded on deposits is less frequent.

Therefore, let’s start with important definitions and particularities of diatom microfossils, such as morphological characteristics of the particles, including where they are most commonly found, and the environmental characteristics that may contribute to their appearance and maintenance in the deposit.

1.1 Types of microfossils and diatom microfossils

According to [1], microfossils are micro-remnants (< 1 mm) of plant or animal species. Microfossils can have various origins, but those found in water, which are significant from an engineering perspective, can be divided into two simple categories based on the chemical composition of their skeletons: calcareous or siliceous. Calcareous fossils include coccoliths and foraminifera, while siliceous fossils include radiolarians and diatoms, which can be seen in Figure 1.

Figure 1.

Types of microfossils in fossiliferous soils. In the group of calcareous microfossils: (a) coccoliths and (b) foraminifera. In the group of siliceous microfossils: (c) radiolarians and (d) diatoms [1].

Regarding diatoms, which are of particular interest in this study, they are unicellular algae of the class Bacillariophyceae that grow in sunlit, saline, brackish, or freshwater environments – whether marine or lacustrine – and consume dissolved silica to build their skeletons [2–4]. The diatom cell wall is silicified, forming its frustule (composed of approximately 95% milky-white silica), making it a microfossil.

The silicification process of diatoms, called biomineralization, begins with the formation of amorphous opaline silica, progresses to the formation of silica polymorphs (tridymite, cristobalite, or quartz minerals, in ascending order of stability), and occurs in nutrient-rich waters (silica, phosphate, nitrate, iron oxide) [1, 5, 6].

Diatoms are subdivided into two orders based on the shape of the frustule: Pennales and Centrales. Members of the order Pennales, the pennate diatoms, have elliptical or rectangular frustules when viewed from the valve and are bilaterally symmetrical with respect to the central axis. Members of the order Centrales, the centric diatoms, have a structure with a centre formed by a single point and may have circular, triangular, or square frustules in the valve view and rectangular or oval frustules in other directions. Diatom cell length ranges from 1 to 2000 µm, but most species fall within 10-100 µm [1, 6, 7].

Diatoms live in almost all types of aquatic and semi-aquatic environments exposed to light and have been classified by salinity preference [8], with pennate species dominating in freshwater and centric species thriving in marine or brackish waters. Examination of diatoms under an optical microscope or a scanning electron microscope (SEM) aids in species identification and, consequently, in determining their likely origin.

Figure 2a illustrates freshwater pennate diatoms identified in Cutiuaú Lake (Amazonas State, Brazil [9]), and Figure 2b presents SEM images of marine and brackish-water centric diatoms identified at Gi Beach (Santa Catarina State, Brazil [10]).

Figure 2a.

Optical and SEM images of pennate diatoms from Cutiuaú Lake, Amazonas: freshwater species: (1–2) Actinella brasiliensis; (3–7) Actinella gracile; (8–10) Actinella guianensis; (11–12) Actinella mirabilis [9]. The freshwater species presented have an elongated shape like a feather, a leaf, or a needle.

Figure 2b.

SEM images of centric diatoms at Gi Beach, Santa Catarina: marine species (A–D) Minidiscus comicus; (E) Cyclotella sp.; (G–H) Actinoptychus splendens; brackish-water species: (F) Discotella stelligera [10]. The brackish and marine water species presented have a shape with a circular disc frustule.

Regarding diatom microfossils, weathering occurs simply when they are deposited in nutrient-poor waters at great depths, given that diatom frustules are prone to dissolution due to pressure at depth or under alkaline conditions (pH > 7) especially in their less robust or weakly silicified forms, such as tridymite or cristobalite minerals [11]. Thus, diatoms must remain in acidic or neutral environments (pH ≤ 7) rich in nutrients (such as silica and iron oxide) so that the silica in their walls is not dissolved and they do not disintegrate. Once disintegrated, free silica can bind to particles, facilitating contact between inorganic soil particles and acting as a cementing agent [12].

Considering these particular characteristics of diatom microfossils, let us review the main aspects of fossiliferous diatomaceous soil deposits, including their geological and microstructural features, geotechnical properties, field behaviour, and associated geotechnical problems.

2. Geological features of the diatomaceous soil deposits

Diatomaceous sediments are frequently observed in volcanically active regions of the planet, as volcanic emissions produce silica-rich aquatic environments that fuel diatom productivity [1, 5, 13–15].

Regions of abundant diatom growth are found in coastal waters, where nutrients are drained from the land or where upwelling occurs – that is, the rise of nutrient-rich subsurface waters to the surface layers of the ocean [16] cited in [14]. An increase in nutrient concentration would boost diatom reproduction to the point that it would generate high biomass and a low-oxygen zone, leading to mass diatom mortality and high-concentration deposition [15]. When they die, the organic matter decomposes, the frustules fossilise and settle at the bottom of the water body, and their skeletons accumulate with mineral sediments, forming diatomaceous soils.

The remains of diatoms can also accumulate in large quantities (concentrations greater than 70%) at the bottom of seas, lakes, ponds, and rivers, forming a sedimentary rock or a gelatinous, light grey sludge called diatomite [5, 17, 18]. It is known by various names, including: diatomite, celite, purified siliceous earth, and diatomaceous earth. These microfossils are considered high-quality materials and have economic value. Diatomite is powdery, highly porous, low-density, and has a rough surface, abrasive properties and high adsorption capacity. Therefore, it has been used as a filter, polishing material, and adsorbent [6].

Mapping microfossils by salinity enables the construction of species diagrams that show sea-level variation [19]. The distinction between microfossil species (marine or non-marine), radiocarbon dating (C14), and the relative sea-level curve allows for the construction of biostratigraphy and the interpretation of sedimentary depositional conditions – transgression or regression – as shown in Figure 3 by [6].

Figure 3.

Groups of microfossil species characteristic of each geological time interval are presented, mapped according to their depositional position relative to the coastline (biostratigraphy) and the relative sea-level change curve [6]. LST = regressive system, TST = transgressive system, HST = high-transgressive system.

According to [6], in the transgressive system (TST), a retrogradation environment occurs, in which the supply of terrigenous sediments is low and the rate of sea-level rise is high, leading to erosion of pre-deposited material and/or deposition of coarser material, with poor preservation and selective removal of microfossils, and abrupt overlying of marine microfossil species over terrestrial species. In the high-stand transgressive system (HST), where sea level reaches its maximum elevation and the largest inundation basin, a progradation environment occurs, in which sediment supply (from the river) and sea-level rise are balanced, and thick deposits of the same type of marine microfossil species are formed, accumulating in stacked layers. In the regressive system (LST), characterized by the descent or the beginning of a slow rise in sea level, a progradation environment may occur in which sediment supply exceeds the rate of sea-level rise, leading to a shallow upward shift from marine microfossil species to non-marine species; or an aggradation environment may also occur, leading to equilibrium and stabilization of the shoreline, which will be observed by the vertical stacking of the same species of terrestrial microfossils.

There are several locations worldwide where diatoms have been found in natural marine and lacustrine soil deposits, namely: in California, United States [7]; in Mexico City, Mexico [20–22]; in Japan in Osaka Bay [1, 13, 23], and in Ariake Bay [24–32]; and in Chile [15].

Diatomites are formed in three main regions: the sub-Arctic waters of the Northern Hemisphere, the sub-Antarctic waters of the Southern Hemisphere, and the Equatorial Belt, where waters are nutrient-rich [6]. Diatomite deposits were reported by [4] in the Ring of Fire, from Chile to British Columbia, and from Japan, the Korean Peninsula, and eastern China to Southeast Asia and Australia, as well as parts of eastern Africa and Western and Eastern Europe, while [33] reported the presence of diatoms in sediments in the Antarctic, Pacific, and Indian Oceans.

3. Porosity and microstructure

Soil structure is an important aspect of the geotechnical behaviour of high-moisture clays. Alteration or collapse of the clay structure is generally cited to account for many aspects of the mechanical behaviour of structured clays [34].

As pointed out by [35], the characterisation of diatomaceous soil deposits should at least include microscopic analysis to recognise the presence, concentration, and state of preservation of fossils, thereby enabling an understanding of how these variables affect the project’s performance.

The following diatom microfossil contents in soil masses have been reported in the literature: Osaka Bay (Japan), with more than 50% at the observable surface [23]; Mexico City with 65% [20, 33]; Oregon (USA) with 50–80% [36]; and Bogotá (Colombia) with 50% quartz [37]. The abundance of fossils in a soil matrix can modify its physical, chemical, and geotechnical properties to the extent that the resulting values differ from conventional values [1].

SEM was used by [38] to study the microstructure of various clays that were normally consolidated or slightly pre-consolidated, and the pore spaces were classified into four broad groups: intra-elemental, intra-aggregate, inter-aggregate, and trans-aggregate, with the terms intra-aggregate and inter-aggregate being the most commonly used in the literature. The terms skeletal and intra-skeletal porosity were introduced by [23] to describe the pore network associated with microfossils in the Asian fossiliferous clays, while [12] proposed classifying pore families into four groups:

  1. Inter-aggregate: pore space existing between particle aggregates;

  2. Intra-aggregate: pore space within the aggregates;

  3. Skeletal: pore space referring to the chambers in the wall of the microfossil frustule;

  4. Intra-skeletal: pore space within the microfossil skeleton.

Figure 4 presents SEM images illustrating the pore families, as classified by [12], for the non-fossiliferous sensitive clay from Port-Cartier, Canada, and the fossiliferous clay from Hachirogata, Japan. Figure 5 presents SEM images of Japanese quick clay from Ariake Bay [29], illustrating the role of microfossils in the flocculated and metastable structure of this soil.

Figure 4.

(a) Classical model showing intra-aggregate and inter-aggregate pore spaces in clay from Port-Cartier, Quebec, Canada. (b) Microstructural model showing skeletal and intra-skeletal porosity in clay from Hachirogata, Japan (adapted from [12]). Inter-aggregate pores have dimensions equivalent to silt-sized particles or larger, whereas intra-aggregate pores can be as small as colloidal particles. Intra-skeletal pores generally have the dimensions of the frustule, while skeletal pores are small holes in the frustule wall ranging from 0.5 to 1 µm.

Figure 5.

SEM images of fossiliferous quick clay from Ariake Bay, Japan, (a) at a depth of 3 m; (b) at 16 m, showing its metastable flocculated structure [29]. The presence of dissolved silica in the soil and on the walls of the microfossil’s frustule promotes the flocculation of surrounding clay particles.

The mercury intrusion porosimetry (MIP) technique has been used to examine the microstructure of soils and can provide quantitative information on the distribution of pore sizes within the material. Mercury porosimetry is based on the principle that a non-wetting fluid, such as mercury, does not enter the porous medium unless pressure is applied.

In MIP, pores are compared to capillary tubes, and the applied pressure, P, is related to a pore entry radius, r, via the equation of [39], which is presented in Eq. (1), where σ is the surface tension of mercury and θ is the solid-liquid contact angle:

P= 2 σ cosθrE1

Reference [40] was the first to investigate the pore-size distribution of clays using MIP analysis; he measured contact angles of various clay minerals with mercury and reported θ = 147° for kaolinite and illite. Reference [34], using freeze-dried samples (prepared by rapid freezing in liquid nitrogen, frozen fracturing, and vacuum freeze-drying) to essentially preserve the soil’s initial structure, carried out a combination of SEM and MIP results to describe the microstructure of the St. Marcel sensitive clay, from Champlain, Canada, in both the undisturbed and compacted states, and at different compaction levels. They showed that compaction of the clay results in the collapse of its inter-aggregate pores, leaving the intra-aggregate pores nearly intact.

Considering diatomaceous soils, a large percentage of diatom frustules consists of empty space (approximately 60-70%, according to [41]). Reference [42] showed that the disk-shaped diatom frustule comprises three membranes at the nanoscale – namely, the areola, cribrium, and cribellum, as illustrated in Figure 6. Reference [43] reported that these membranes contain large (d > 2 um), small (0.2 < d < 0.6 um), and tiny (d > 2 um) nanoholes, following the study by [44] on pore-size classification using MIP.

Figure 6.

Microstructural perspective of a disk-shaped diatom microfossil [43]. Two siliceous shells are embraced by a girdle band. In this system, three membranes with different orders of magnitude of interconnected pores are seen: areola, cribrium, and cribellum.

Reference [23] observed that inter-aggregate and intra-aggregate pore spaces (with diameters smaller than 0.5 µm) are the first to disappear when the soil is compressed. According to [1, 12] the diameter of diatom skeletal pores can be as small as 1 µm and as large as 100 µm for the intra-skeletal pore diameter. The skeletal pore space is easily recognised in a MIP logarithmic-differential curve as a set of micropores with radii ranging from 0.02 to 0.1 µm. These pores do not disappear during the compaction of these soils. Intra-skeletal pore space, on the other hand, varies greatly depending on the type of microfossil present and can have typical radii ranging from 10 to 50 µm. Under vertical compression, these are the last pores to close and become the pore family with the largest radius. Only under high compressive pressures do they shrink drastically in size or close, due to the crushing of diatom microfossils.

SEM and MIP analyses were used by [31] to evaluate changes in the microstructure of natural diatomite from Oita, Japan, with increasing consolidation pressure, as illustrated in Figure 7, and observed that the microstructure of the diatomite remains unchanged at pressures lower than the apparent pre-consolidation stress (500 kPa) and changes significantly under virgin compression, especially at pressures greater than 1,500 kPa.

Figure 7.

Cumulative (a) and logarithmic-differential (b, c, d) distributions of diatomite pore sizes with increasing compression pressure (adapted from [31]). At pressures exceeding the pre-consolidation pressure of 500 kPa, greater destruction of the soil structure is observed, involving the closure of inter-aggregate macropores and intra-skeletal pores due to the crushing of microfossils.

4. Geotechnical properties

4.1 Characterisation and index properties

Diatomites consist mostly of silt-sized diatom microfossil particles, have a high natural water content (30–80%), and a low dry bulk density (ρd of 0.64 to 1.36 g/cm3), characteristics that, in theory, should result in poor engineering properties [7]. The grain density (ρs) of diatom microfossils is very low, ranging from 2.23 to 2.38 g/cm3 [5, 14]. Soil with a significant diatom content (e.g., > 15%) will have a reduced ρs, with Ref. [5] highlighting a strong correlation between the diatom microfossil content and the specific gravity, Gs, and, consequently, ρs.

In terms of consistency, in clayey soils the plasticity is directly associated with the specific surface area, SSA. However, in the diatomite case, when pure (without association with soil fine particles), it lacks plasticity [5, 14], and the SSA ranges from 10 to 20 m2/g [1], which is high for non-clayey inorganic particles. Reference [13] noted that in some Japanese soils, the SSA does not increase with the plasticity index as expected, a fact attributed to the presence of diatoms.

When diatomite is associated with clayey soil, it has been shown to increase wL and wP values. The influence of diatomite on the wL and wP of mixtures of kaolin and diatomite, with 0%, 25%, 50%, 75%, and 100% diatomite by dry weight, was studied by [23], and it was observed that increasing diatomite content tends to significantly increase wL and wP, whereas PI remains constant or is slightly reduced. Since the clay content decreased as the diatomite content increased, the colloidal activity of the mixture also increased from 0.5 (100% kaolin) to a value close to 1.5 (with 75% diatomite and 25% kaolin). Three types of mixtures: kaolin and crushed Toyoura sand; kaolin and diatomite; and natural kaolin clay from Singapore with diatomite, with different coarse-particle contents, were used by [5] to obtain the Atterberg limits. The mixtures of kaolin with crushed Toyoura sand behaved as expected in conventional soil mechanics for ordinary soils, with the limits decreasing as the coarse particle content increased. However, for mixtures with diatomite, increasing its content increased the Atterberg limits. Diatom microfossils can retain large amounts of water within their porous skeletons, introducing a systematic error in index properties, since they do not reflect the contribution of water adsorbed only on clay particles.

Figure 8 presents the Casagrande plasticity chart used in the Unified Soil Classification System, USCS, produced by [35].

Figure 8.

Casagrande plasticity chart of diatomaceous soils from different origins. Numbers beside symbols indicate the percentage of diatomite (adapted from [35]). (1) USA, reference [45]; (2) Japan, reference [5]; (3) Mexico, reference [46]; (4) USA, reference [47]; (5) Mexico, reference [48]; (6) Colombia, reference [48]; (7) Japan, reference [29]; (8) Germany, reference [14]; (9) China, reference [49]. The increase in diatom content in the soil always increases the wL and shifts the USCS classification from line A toward MH (high-plasticity silt) and OH (high-plasticity organic soil), even if the soil contains no significant organic matter. The behavior of the PI varies depending on the clayey soil, the type of microfossil, and its content.

It comprises mixtures of diatomite in different proportions with natural soils and kaolinitic clay. The increase in diatom content in the soil shifts the USCS classification from line A toward MH (high-plasticity silt) and OH (high-plasticity organic soil), even if the soil contains no significant organic matter. Clearly, in all cases, the wL increases with the diatomite content. However, the PI increases, remains nearly constant, decreases continuously, or increases again after decreasing as the diatomite content increases. As reported by [15], specifically for mixtures of diatomite and kaolinite, patterns such as those indicated by numbers 1, 3, and 4 in Figure 8 were also observed by [5, 50], and [51], respectively.

The variable response of the consistency limits to changes in diatomite concentration in soils (with corresponding USCS reflexes shown in Figure 8) may be due to several factors. For instance, considering two diatom species – Aulacoseira granulada (cylindrical shape) and Coscinodiscus centralis (disk-shape) – mixed with kaolin, it has been observed [48, 52] that the size and shape of diatom microfossils, as well as the roughness and angularity of these particles when broken and interlocked, affect their geotechnical characterisation and, more importantly, hydro-mechanical behaviour. Thus, the evaluation of geotechnical properties of this soil type, following conventional geotechnical characterisation standards, must be seen with caution.

The liquidity index (LI), defined by [53] and calculated using Eq. (2), is an important parameter for comparing the consistency of different clays and correlates well with their compressibility, strength, and sensitivity.

LI= w−wPwL−wPE2

When the water content is well above the liquid limit, the LI value exceeds 1.0, indicating that the soil may behave as a soft, viscous clayey soil. For instance, in Canadian sensitive clays, LI values greater than 1.2 have been considered capable of causing a slope failure to become a flowslide (e.g., [54]).

Table 1 presents geotechnical characteristics and index properties of diatomaceous soils. In general, diatomaceous soils have natural water contents, as well as wL and wP higher than those of non-diatomaceous soils. LI greater than 1.2 was observed in normally consolidated diatomaceous clayey soils in Ariake Bay, Japan [29] and in the Port of Santana, Amapá, Brazil [55], with the difference that the marine-origin Ariake clay has been leached by fresh water over geological time. It should be noted, however, that, besides the leaching of salts associated with this Japanese clay, the origin of quick clays has been linked to glacial depositional conditions, with some properties different from those of diatomaceous quick clays from Ariake Bay, Japan.

Location Depth ρs w wL wP PI LI Clay SSA Source
(m) g/cm3 (%) (%) (%) (%) (%) m2/g

Mexico City, Mexico

–

2.35

500

500

150

350

1

20–30

–

[20]

Osaka Bay, Japan

99.2*

–

57

79

31

48

0.6

46

119

[23]

Kansai airport, Osaka Bay, Japan

383*

2.71

43

117

42

75

0.01

19

120

[12]

Hachirogata, Japan

6

2.49

179

221

71

150

0.7

15

225

Ariake Bay, Japan

3-12

2.60–2.63

120–150

105–130

30–70

60–100

1.2–1.5

50

105

[29]

12–16

2.60–2.63

90–100

65–95

25–45

40–50

1.1–1.7

50

–

Port of Santana, Amapa, Brazil

20–22

2.68–2.75

48–55

48–63

25–32

23

0.7–1.0

35

69–77

[55]

36–39

2.63

55–82

50–61

33–41

17–19

1.2–2.1

33–39

54

El Dorado airport, Bogotá, Colombia

25

–

66

76

42

34

0.71

24

–

[52]

Variable

–

2.00-2.60

–

80-500 +

40-150

40-350

–

–

10-100

[59]

Diatomite Aulacoseira, Colombia

–

2.55

–

99

48

51

–

23

–

[48]

Diatomite Coscinodiscus, Mexico

–

2.34

–

132

–

–

–

0.7

–

Diatomite Mejillones Bay, Chile

–

2.63

–

82

44

38

–

18-20

–

[15]

Diatomite ChangbaiPrefecture, China

–

2.25

–

175

153

22

–

25

180

[49]

Piccolo Sea, Italy

0.5

2.70

155

114

44

70

1.59

30

–

[60]

Table 1.

Geotechnical characteristics and index properties of diatomaceous soils.

*Depth below sea/river/lake level.


4.2 Compressibility

Natural clay soils differ from reconstituted soils (typically comprising laboratory-prepared specimens with the same water content and total specific mass) in several important ways. The main difference stems from the influence of soil structure (fabric, including pore size and distribution), interparticle forces, cementation, and ageing. By studying the behaviour of reconstituted clays, depositional and post-depositional factors present in natural sedimentary deposits are eliminated, focusing on the intrinsic properties of the soil, typically with a given mineralogy, water content, porosity, and pore fluid.

For fine-grained soils composed of a high percentage of clay minerals, compressibility depends fundamentally on physicochemical factors (primarily the repulsive force between particles), which depend on the colloidal properties of the clay and the chemical composition of the solution in the soil pores [56].

The compression index, Cc, defined as the change in the void ratio per 10-fold increase in vertical stress for saturated clayey soils, correlates very well with the plasticity index: Cc = Gs. PI/2or PI/74 (assuming Gs= 2.70), as well as the recompression index, Cs, Cs= PI/370, with Cc/Cs ≈ 5 [57]. Cc increases in the order: Cc kaolinite < Cc illite < Cc montmorillonite, and ranges from 0.19 to 0.28 for kaolinite, 0.50 to 1.10 for illite, and 1.0 to 2.6 for montmorillonite [58]. Reference [56] suggests evaluating the relative importance of osmotic double-layer repulsive forces on the compressive strength of clayey soils using the ratio a = Cs/Cc. Clayey soils and highly compressible coarse-grained soils have “a” as low as 0.1 and are dominated by mechanical forces. The value of “a” increases for pure clays highly dominated by physicochemical forces. In extreme cases, it can reach 1.0.

For coarse-grained soils and mixtures of these with clays, compressibility also depends on direct particle contact and on the mechanical properties of the grains that constitute them.

On the compressibility of natural diatomaceous soils, Ref. [31] evaluated Oita diatomite and Ariake quick clay, both from Japan, using oedometer loading up to high stresses, with the resulting compression curves shown in Figure 9. Ariake clay, has a water content of 126%, is normally consolidated, is highly sensitive (classified as quick clay by [25]), has high compressibility under virgin compression (Cc = 1.3), and is considered diatom-structured. For Oita diatomite, with a water content of 141%, the effective vertical stress in the field is estimated at 39.3 kPa; however, the material exhibited an apparent pre-consolidation stress of 2,100 kPa, yielding an OCR of 53. The authors attributed this to the silica cement between the diatom grains, which creates strong bonding and imparts the material’s highly structured characteristics. The high compressibility of the material under high effective vertical stresses (Cc = 3.5) was attributed to the breakage of diatom microfossils (viewed in SEM images).

Figure 9.

Oedometer compression curves of Oita diatomite and Ariake clay in the undisturbed and remolded states [31]. The presence of diatom microfossils in the soil implies an increase in the initial void ratio (> 3.0) and apparent preconsolidation stress, especially in Oita diatomite (2100 kPa).

Diatomaceous sediments in Japan with a disordered microstructure were under-consolidated to depths of approximately 400 m below sea level due to water retained by the microfossils [61]. It was proposed by [62] that the compressibility of sediments containing large quantities of microfossils is greater because of water retained within the skeletons, and that a sudden release of this water would decrease effective stress and lead to further consolidation. Reference [63] confirmed that microfossil-rich sediments were frequently found in a normally consolidated state or undergoing consolidation. In addition, Ref. [23] noted that diatom microfossils are deformable under high effective stresses and that clay particles tend to align around the deformed microfossil, indicating a possible arching effect that, in turn, could protect the microfossils from further deformation.

The lacustrine soft diatomaceous soil from Bogotá, studied by [64], exhibited high void ratios and compressibility, which were well related to the liquid limits, whose values increased with diatom content. Eqs. (3) and (4) present the relationship obtained for the compression index, Cc, and the recompression index, Cs, respectively. In addition, the Burland’s intrinsic compression line (ICL) described the normally consolidated behaviour of this soil with high accuracy (R2 = 0.86), providing a normalised void index, Ivn, given by Eq. (5). Furthermore, following [65], a relationship between the void ratio at 100 kPa (e100) and the liquid limit (wL) was proposed for this soil, as given by Eq. (6).

Cc= 0.01 (wL−0.58) E3
Cs= Cc 7.6 E4
Ivn= 2.45−1.285logσv′+0.015 (logσv′)3E5
e100= 0.0158 (wL+35) E6

Table 2 shows values of the relationship between the compression index, Cc, and the expansion index, Cs, for other soils found in the current literature.

Location Gs w (%) eo σ’p (kPa) OCR Cc Cc/Cs Source

Oita diatomite, Japan

2.13

141

3.01

2100

53.4

2.62

190

[31]

Ariake clay, Japan

2.65

126

3.35

42

1.1

2.80

81

Diatom (A), Chile

2.15

104

2.24

800

–

1.00

15

[66]

Diatom (B), Chile

68

1.47

800

–

0.70

20

Mejilones Bay, Chile

2.64

96

3.01

450

–

1.63

16

[15]

83

2.44

620

–

1.77

15

63

2.12

700

–

1.28

37

62

3.05

650

–

2.04

29

Table 2.

Relationship between Cc and Cs for different diatomaceous soils.

Reference [7] observed in oedometric consolidation tests of a diatomaceous earth landfill in Southern California, USA, with ρd = 0.99 g/cm3 and w = 52.2%, that under effective vertical stresses less than 50 kPa, it was practically incompressible, but under high stresses (1600 kPa), compressibility increased substantially, as illustrated in Figure 10a. The author also noted that the consolidation curve of the diatomaceous landfill (Figure 10b) does not have the characteristic Terzaghi shape and that it is difficult to determine the end of primary consolidation for this material. The compressibility of this soil is governed by diatom microfossils (at a concentration of 65% by dry weight), and its high compressibility under high effective vertical stresses is attributed to their crushing.

Figure 10.

Compressibility of diatomaceous fill [7]: (a) Under effective vertical stresses less than 50 kPa, the diatomaceous fill was practically incompressible, but under high stresses the compressibility increased substantially. (b) The diatomaceous fill does not have the characteristic Terzaghi shape in the consolidation curve and that it is difficult to determine the end of primary consolidation.

Reference [5] evaluated the compressibility of mixtures of reconstituted kaolinitic clays [kaolin (K) and Singapore clay (SC)], with granular materials [Japanese diatomite (D) and crushed Toyoura sand (T)], using conventional oedometric consolidation tests, and typical results are illustrated in Figure 11. To avoid initial effects, the authors evaluated the compression indices (Cc) of the mixtures in the linear portion of the curves under high effective vertical stresses.

Figure 11.

(a) Typical e-log p curves for mixtures containing 25% granular material (diatomite (D) or crushed Toyoura sand (T)); (b) Compressibility characteristics of the mixtures as a function of granular material content [5]. The compressibilities of kaolin and Singapore clays increased sharply with the addition of diatomite, unlike Toyoura sand (an increase in the content of which reduces soil compressibility).

The results presented in Figure 11b show that the compressibilities of kaolin and Singapore clays increased sharply with the addition of diatomite; Cc increased with diatomite content from 0.5 to 1.5, whereas the addition of crushed Toyoura sand had the opposite effect, as expected from classical soil mechanics. This behaviour followed the same trend observed for the Atterberg limits determined for these mixtures. Similar results were obtained by [50] and [67] in mixtures of kaolin and diatomite from Mexico.

4.3 Hydraulic conductivity

The mineralogical composition, grain size and distribution, porosity, fabric, and pore-fluid characteristics determine the material’s structure and influence the hydraulic conductivity, k, and the consolidation coefficient, cv.

Within the water content range between wP and wL, the hydraulic conductivity of clay minerals is less than 1 × 10−7 m/s and can vary to values lower than 1 × 10−12 m/s for some montmorillonites with monovalent ions, with the range for natural clay soils varying from 1 × 10−8 to 1 × 10−10 m/s. For clay minerals compared at the same water content, the magnitude of hydraulic conductivities is in the order: kmontmorillonite < killite < kkaolinite. Dispersed clays settle with the particles’ longitudinal axes nearly parallel to the bed. This generates anisotropy, with the horizontal hydraulic conductivity much greater than the vertical. Flocculated clays tend to settle with a more isotropic structure, resulting in k values that are more similar in both directions and higher overall. This is because hydraulic conductivity is associated with macropore volume, which is greater in a flocculated structure. The consolidation coefficient, cv, is also related to soil composition, since cv is directly proportional to hydraulic conductivity and inversely proportional to the compressibility coefficient, av. The values of cv determined in the [58] study ranged from 0.06 x 10⁻⁸ to 0.3 x 10⁻⁸ m2/s for montmorillonite, 0.3 x 10⁻⁸ to 2.4 × 10 8 m2/s for illite, and 12 × 10⁻⁸ to 90 × 10⁻⁸ m2/s for kaolinite.

For the Ariake quick clay in Japan, which contains diatoms and smectite in its composition [29], found cv of 100 cm2/day, i.e., 1.2 × 10⁻⁷ m2/s, considered a high value and associated with the presence of diatom microfossils in the soil and the flocculated structure. Regarding hydraulic conductivity relative to the field void ratio, the Ariake clay exhibited values ranging from 6 × 10⁻⁸ to 4 × 10⁻⁷ cm/s.

Regarding the diatomite-clay mixtures in the Ref. [5] study, the results of oedometric compression tests shown in Figure 12 indicate that the addition of diatom microfossils significantly increases the soil void ratio and, consequently, the hydraulic conductivity.

Figure 12.

Hydraulic conductivity (k) x void ratio (e) in mixtures with diatomite under a pressure of 640 kPa [5]. The addition of diatom microfossils (D) significantly increases the soil void ratio and, consequently, the hydraulic conductivity, especially with D > 75%.

Reference [68] also studied the effects of diatom particles on the compressibility and permeability behaviours of diatom–kaolin mixtures. They observed that compression and permeability behaviours of these soils are under control of both microstructural characteristics of diatom (including shape, surface morphology and pore structure) and mixtures (matrix structure or skeletal structure depending on the diatom proportion), as well as the evolution of these properties under compression, which is different from the case of conventional fine-grained clays, controlled by particle composition, pore features and pore fluid.

4.4 Shear strength

Following [69], it would be expected that the angle of internal friction, ɸ’, of clayey soils decreases with an increase in the plasticity index. However, on the shear strength of diatomaceous soils [20], identified ɸ’ = 40° for a clay from Mexico City, even with a plasticity index of about 300%, and attributed this value to the presence of a large number of diatoms in the soil (65% by weight).

In CIU-type undrained triaxial compression tests [7], sheared two diatomite soil specimens under the following conditions: (1) ρd = 1.16 g/cm3, w = 41.2%, and effective confining pressure (σc’) of 13.8 kPa; and (2) ρd = 1.23 g/cm3, w = 38.5%, and σc’ = 27.6 kPa. This author found ɸ’ = 44° and a zero-cohesion intercept, attributing these strength parameters to the rough surface of the diatoms, particle interlocking, and low effective testing stresses.

Reference [5] found, through direct shear test results at constant volume, that the ɸ’ of mixtures of kaolin and Singapore clays with diatomite increased with increasing the microfossil content, reaching a maximum value of 43° for pure diatomite, and that the results for the mixtures were higher than those predicted by the ɸ’ x PI relations, as was also the case for natural Japanese diatomaceous soils. Figure 13 by [1] presents these results.

Figure 13.

Effects of diatom microfossils on the internal friction angles (ɸ’) of soils in relation to the plasticity index [1]. The numbers 0, 25, 50, and 75 within the image represent the diatom concentrations (D). A concentration of D = 25% causes a considerable increase in the ɸ’ without significantly altering plasticity, whereas higher contents further increase ɸ’ but also result in lower plasticity indices.

Reference [33], using a single-shear apparatus at constant volume (NGI type) with specimens prepared at OCRs = 1 and 2, observed that the ratio su/σ’v (the ratio of the undrained shear strength, su, to the effective vertical stress, σ’v) also increased. The chemical bonds of silica, promoted by interactions between microfossils, mineral particles, and pore water, produce a cementing effect that increases su. The same trend of increasing ɸ’ with diatom content through direct shear tests on mixtures of kaolin and diatomite was observed by [14]; however, they obtained a maximum ɸ’ of 32° and a cohesive intercept of 69 kPa for diatomite (σ’v from 200 to 600 kPa).

Reference [64] studied the soft diatomaceous soil of the Bogotá lacustrine soil using a wide variety of laboratory tests and proposed practical correlations for this type of soil, considering the mobilisation of shear strength. Eq. (7) presents the relationship between the plasticity index, PI, as an intrinsic parameter, and water content, w, as a state parameter, with the undrained shear strength (su/pa, where pa is atmospheric pressure), and Eq. (8) presents the relationship found between the critical friction angle (ɸcrit) and the liquid limit (wL).

supa= 14 PI w−0.18E7
Φcrit= 18.5+0.112 wLE8

S-wave and P-wave propagation was investigated by [70], concluding that the presence of diatom microfossils, which are porous and fillable, can soften the soil matrix, thereby attenuating wave propagation and reducing shear stiffness at low deformations.

Reference [71] studied the load and displacement trajectories of shallow and deep foundations in artificial diatom-clay mixture soils with different diatom genesis, using reduced-scale models installed in a small geotechnical centrifuge. The addition of diatoms significantly increased the bearing capacity of a clay soil. The interlock offered at the microstructural level by the interaction between frustules of Mexican diatoms, due to their disk shape, was greater than that of frustules originating in Colombia, which have a cylindrical shape and are characterised mainly by their water retention.

Reference [35] presented the variation of the friction angle (ɸ) with the diatom content (%) in soils of different origins, illustrated in Figure 14. They observed that not only the content but also the diatom species, and consequently its shape and the microstructure formed in the soil, affect measurements of the physical and mechanical properties of diatomaceous soils of different origins.

Figure 14.

Friction angle (ɸ’) x diatom content (%) in soils of different origins (adapted from [35]). (1) USA, Multispecies, reference [45]; (2) Mexico, OCR = 1, reference [33]; (3) Mexico, OCR = 2, reference [33]; (4) Mexico, Coscinodiscus centralis, reference [48]; (5) Colombia, Aulacoseira granulata, reference [48]; (6) Japan, Centric monospecies + kaolin, reference [5]; (7) Japan, Centric monospecies + Singapore clay, reference [5]; (8) Centric monospecies OCR = 1, reference [46]; (9) Centric monospecies OCR = 2, reference [46]. The content of diatom species, and also its shape and the microstructure formed in the diatomaceous soil affect the measurements of ɸ’.

4.5 Field behaviour and associated engineering problems

As noted by [59], published literature is scarce addressing in situ test interpretation, geotechnical design procedures, and construction methods for diatomaceous soils.

Construction on diatomaceous soils has resulted in unexpected engineering consequences. Reference [35] mentions construction processes that encounter problems such as differential settlements, pile rebounds, and irregular pore pressures caused by frustules, and highlights that the range of possibilities for the development of stable construction on a diatomaceous soil deposit is uncertain, posing a risk of loss of life when buildings, bridges, embankments, slopes, or other works are executed. Sometimes, this situation leads to the reprocessing of research and consulting projects, the reconstruction of works or the need for emergency attention.

Regarding engineering problems encountered in association with diatomaceous soils, one example related to compression behaviour is excessive settlement in embankment structures due to secondary consolidation after the construction of the foundation embankment at Wickiup Junction, in the “La Pine” basin, Oregon, USA [36], which happened in response to deformation and crushing of skeletal particles under the imposition of new loads.

Examples of foundation problems in diatomaceous soils include excessively long piles or pile rejection at shallow driving depths in Klamath County, Bonanza, Oregon, USA [72, 73]. Reports by [74, 75] on the soil response in the Cooper Marl sector, Charleston, South Carolina, USA, highlighted the difficulties of pile driving in diatomaceous soils due to the unexpectedly high rebound observed during driving of a closed-end tubular pile, with rejection occurring at a depth of less than 5 m, followed by the driving of open-end piles.

Considering slope failures in diatomaceous sediments, these may have generated destructive tsunamis, as reported by [76–79]. Susceptibility to geological hazards in road projects has also been reported in the Tengchong region, Yunnan, China [80], and liquefaction failures (even when index properties indicate otherwise) have been reported in underwater landslides of diatomaceous deposits in Lake Villa-Rica, south-central Chile, affecting infrastructure both on land and at sea [49]. Furthermore, Ref. [14] reported that submarine slope failures, with an inclination of less than 1°, were identified in marine diatomaceous sediments.

5. Final comments

Based on relevant information from the current literature, this chapter presented basic considerations on characterisation and its relationship to the geotechnical behaviour of different diatomaceous sedimentary soil formations, diatomite types, and soil-diatomite mixtures, highlighting the complexity of these fundamental aspects, which, in most cases, do not follow conventional Soil Mechanics expectations. Only a few recent studies have been conducted to improve the knowledge about this geological material that is not covered in this chapter, especially regarding the influence of the size and shape of microfossils on the values of physical and geotechnical properties obtained, the water retention capacity of microfossils, the influence of microfossils on results of field tests and the effects of diatom particles on soil behaviour under unsaturated and cyclic loading conditions. Suggestions for research have focused on techniques for stabilising diatomaceous soil masses and on appropriate construction methods that avoid triggering mass movements due to structural metastability. In fact, diatomaceous soil deposits are intriguing and pose challenges for proper geotechnical engineering understanding, being potentially classified as a difficult or problematic soil type and therefore requiring further basic research and a case-by-case approach in practice.

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Written By

Elaine C. G. Barreto and Tácio M.P. de Campos

Submitted: 16 May 2026 Reviewed: 08 June 2026 Published: 25 August 2026

© 2026 The Author(s). Licensee IntechOpen. This content is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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